Inductive tuner and contact therefor



June 5,1945. w. H. KUNZ 2,377,789

INDUCTIVE TUNER AND CONTACT THEREFOR Filed March 24, 1942 z 41 1a 1a 14 INVENTOR. 34a Walden fi lfuuz X, 1g -KMM I Patented Juno 5, 1945 INDUCTIVE TUNER AND CONTACT THEREFOR Walden ll. Knnz, Indianapolis, Ind., assignor to P. R. Mallory a (30.. Inc., Indianapolis, Ind., a corporation of Delaware Application March 24, 1942, Serial No. 438,007

2 Claims.

This invention relates to variable inductance devices of the slide-wire type and to contacts thereior. I

An object of the invention is to improve slidewire variable inductance devices and the contacts therefor. I

Other objects of the invention will be apparent from the following description and accompanying drawing taken in connection with the appended. claims.

The invention comprises the features of construction, combination of elements, arrangement of parts, and methods of manulacture and operation referred to above or which will be brought out and exemplified in the disclosure hereinafter set forth, including the illustrations in the drawing.

In the drawing:

Figure 1 is a side elevation, partly in section of a slide-wire variable inductance device embodying features of the present invention;

Figure 2 is a section on the line 2-2 of Figure 1: l

Figure 3 is a plan view of a contact comprising part of the inductance device;

Figure 4 is an end view of the contact; and

Figure 5 is a section through a contacting portion of a modified contact and the wire of the inductive coil.

Slide-wire variable inductance devices are used for tuning radio receivers, oscillators and various electric and electronic instruments and devices. Generally they comprise a bare wire coil wound on a suitable insulating form which is mounted on ashaft so as to rotate about its axis. Circuit connection is made to one end of the coil by a contact brush sliding on a suitable slip rin mounted on the end of the coil form and by a sliding contact which slides along the wire of the coil as it is rotated and simultaneously engages a stationary trolley rod mounted alongside the coil form.

Most of the sensitive electrical circuits in which such devices are used require that the sliding contact make good and positive contact at all times and slide along the wire of the coil without momentary interruptions .of contact which would cause disturbances in the'circuit. The contact must also be free from any susceptibility to vibrations or jarring which might cause "microphonics" in the circuit. Mechanically the contact must be suillciently light to permit easy rotation of the coil to eifect the desired tuning and must produce only the minimum possible amount of wear on the contact and coil surfaces so as to permit many thousand cycles of operation. The contact pressure must be low enough to prevent excessive wear between the contacting surfaces and still must be positive and not readily subject to fouling by any small particles of dirt or foreign matter which mi t become deposited on the surface of the coil, or to circuit interruption due to minor nicks or irregularities on the surface of the coil wire.

Some of the mechanical and electrical requirements are conflicting. For example, positive and constant contact is required for electrical reasons while on the other hand, lightness of contact/is required for ease of tuning and to keep down wear over long periods of use. Therefore, the problem of providing a suitable contact, which might at first appear relatively simple, is, in fact. a very complex and difilcult one. The problem becomes increasingly difllcult with high frequency-circuits where comparatively small inductances are used and all parts adjacent to the coil or circuit ailect its performance.

The present invention contemplates a sliding contact for slide-wire variable inductance device which is extremely simple in construction and still embodies substantially all the features desired of a contact for this purpose. The contact comprises a single piece of spring material having contacting nibs at its ends engaging the coil and also a middle contact portion for engaging the trolley rod parallel to the side of the coil.

While a preferred embodiment of the invention is described herein, it is contemplated that considerable variation may be made in the method of procedure and the construction of parts without departing from the spirit of the invention. In the following description and in the claims, parts will be identified by specific names for convenience, but they are intended to be as generic in their application to similar parts as the art will permit.

Referring to the drawing, the slide-wire variable inductance device shown in Figures 1 and 2 comprises a pair of-spaced parallel end plates Ill and H held in spaced parallel relation by longitudinalspacer rods III. The unit is enclosed in a metal can shield i4 and may be mounted in the apparatus in which it is to be used by a flanged mounting base i5 welded to the outside of the shield. A rotatable control shaft l8 extends axially through the shield and is pivoted in end plates I 0 and i I for rotation by a control knob ii. The shaft carries a coil form l8 of tubular insulating material such as resin impregnated fibre.

Bare wire coil I3 is wound on the surface of form 3 with adjacent turns spaced from each other. If desired, the form may be provided with a shallow groove in which the wire I3 is wound. Coil 3 may suitably be made of hard drawn copper wire plated with silver, and, if desired, coated with a protective lubricant. The ends of the coil N are soldered to metal caps and 2| attached over the ends of the coil form. The solder may be deposited at the ends of the coil in sufiicient quantity to provide enlarged stops 22 and 23, if desired, to limit the rotation of the coil when the contactor reaches the end of its travel. End cap 20 has a central aperture spacing it from shaft l6 so that it is insulated therefrom. A fork-shaped contact spring 24 is mounted on a suitable insulating block 25 inside end wall l0 and carries a pair of contact brushes 26 on the ends of its fork arms which slide on can 20, thereby providing a slip ring type connection to the end of the coil. Contact spring 24 extends out through a suitable aperture in the shield hi to provide an external circuit connection. End cap 2| is positively grounded to the frame of the deviceby contact spring 21 secured to end wall I l and carrying contact brushes 28 sliding against cap 2|. Mounting base I5 is provided with a punched down grounding lug 23.

Trolley rod 30 extends along the side of coil form I8 and is supported and electrically connected at its ends to end walls HI and H. Sliding contact element 3| engages coil I3 and trolley rod 30 to provide a movable connection for varying the amount of coil IS in the electric circuit. That portion of coil l3 to the right of contact 3|, as shown in Figure l, is grounded to the frame at its ends by contacts 28 and 3|, respectively, and hence is effectively removed from the circuit.

A featureof the present invention resides in the shape and construction of contact element 3| which is illustrated in'detail in Figures 3 and 4.

Contact 3| comprises a stamping formed from sheet spring metal, such as'phosphor bronze or oher spring metal of suitable electrical and mechanical characteristics. Contact element 3| comprises a central body portion 32 of generally rectangular shape bent to form a channel across the middle of the contact element and having a rectangular aperture 33 cut out of its center to increase its compliance. The two nibs 34 and 35 of the contact element are relatively narrow portions of uniform width which are bent to form grooves or channels extending longitudinal of the nibs, the grooves being bent around a radius substantially equal to the radius of the wire of coil I3. The grooves are on the opposite face of contact element 3| from the channel formed in mid-portion 32 and it is obvious that their axes are substantially at right angles to the axis of the channel in the mid-portion of the contact. A slight skew may be used, if necessary, to allow for the pitch angle of the coil |3. The nibs 34 and 35 are joined to the edges of the channel in mid-portion 32 by tapered connecting legs 33 and 31 bent down from the edges of central channel portion 32. Contact element 3| is thus adapted to be interposed between trolley rod 30 and coil I! with the channel portion 31 slidable along the trolley rod and the nibs 34 and 35 both slidable along the wire of coil l3. Nibs 34 and 35 rest on the same turn of the coil at spaced points thereon as clearly indicated in Figure 2. The contact element 3| is originally bent sufliciently to provide the necessary spring pressure on the nibs when the element is interposed between the rod 30 and the coil.

The radius of curvature of mid-portion 32 is slightly greater than the radius of trolley rod 33 so that actual contact with the rod is made along a line extending along the middle of the channel portion. This arrangement eliminates instability of motion of contactor along the trolley rod by precluding any tendency of the contactor to grab" the trolley rod. It will also be noted that a tangent line at the point of contact between either nib and the coil passes through or, very close to rod 30. Since any forces on the nib tending to displace the contact element during rotation of the coil are along this tangent line, it will be seen that this arrangement produces dynamic stability of the contactor and prevents any tendency for the contact nib to jump or jam when the coil is rotated.

In operation, when the coil is rotated the nibs slide along the wire and move the contact element 3| along rod 30. If the coil is rotated sufficiently to bring contact element 3| to the end of the coil, the leading nib will engage stop member 22 or 23 to prevent further rotation. Of

course, in some instances it is contemplated that other stop means may be provided, such as 8. Geneva gear mechanism secured to one end of shaft I8.

In the modification illustrated in Figure 5 a nib 340 having a V-shaped channel is used in place of rounded channel nibs 34 and 35. In this case nib 34a makes contact with the wire I! of the coil at the two points of tangency.

It is obvious that two or more variable inductances can be mounted on the same control shaft or that several devices can be ganged together under a common control. It is also contemplated that in some instances coil l9 may be stationary and the trolley rod and contact 3| be rotated about it.

From the foregoing description it will be evident that the present invention provides a con tactor formed of a single sheet metal element. By reducing the size and number of parts of the contactor, the circuit inductance and capacitance can be considerably reduced and the efllciency of the inductance element improved. These features are highly desirable in high frequency circuits.

The contact element is smooth in operation, easily assembled and adjusts itself automatically to the spacing between the coil and the trolley rod, thus compensating for any variations in spacingdue to eccentricity or irregularity in the coil form. The contact is remarkably free from microphonic tendencies and is free from any tendency to jump or jam as a result of rotation of the coil. It has the effect of dislodglng for eign matter from the contacting surface of the wire and rides smoothly over small nicks or irregularities in the wire surface. Due to its light weight and method of mounting, it is not readily. affected by extreme vibration such as may be encountered in aircraft radio service.

Due to the small size of the contact element, the size of the entire tuning device may frequently be reduced both in length and diameter since large clearance space such as might be required for a bulky contactor is unnecessary. Due to its small size and comparatively short arm length the contact is free of any loose play or backlash and permits accurate setting and resetting of the contact position. The contact permits use of the entire length of the coil without any wasted end section.

The contact pressure is very constant over the mounted for rotation about entire coil path and wear on the coil and the contact surfaces is kept at a minimum.

While the present invention, as to its objects and advantages,' has been described herein as carried out in specific not desired to be limited thereby but it is intended to cover the invention broadly within the spirit and scope of the appended claims.

What is claimed is:

l. A variable inductance device comprising in combination a bare helical conductive coil its longitudinal axis, a trolley rod mounted parallel to the side of the coil and ically balanced shape, being bent into smooth-surfaced grooved contact nibs engaging the wire of said coil in the grooves therein and the mid-portion of said element being bent into a smooth-surfaced transverse channel which is concave on the opposite face of said grooves and engages the trolley rod, said contact elementbeing biased to produce embodiments thereof, it is smoothly sliding and uniformly distributed spring pressure against said coil at said nibs and against said rodat said channel.

2. A slide wire variable inductance device comprising in combination a coil unit rotatable about a longitudinal axis constituted of a form or cirunder compression trolley rod integrally sheet spring metal shape, the ends of said member being bent into downwardly concave smooth-surfaced grooved nibs slideably engaging said wire at two spaced points and the mid-portion or said member being bent into an upwardly concave smooth-surfaced transverse channel slideably engaging said trolley 

